Numerous industrial processes are characterized by problems (to a greater or less degree) with the development of a positive fluid-solid interface, where the fluid is required to completely wet the solid surface. Printing inks on glossy surfaces, tin solders on printed circuit boards, dye solutions on fabrics, and resinous impregnation of fibrous materials, such as glass fiber, graphite, and aramid, are examples where efficient wetting has importance.
Part of the problem is caused by difficulties associated with an economically viable speed of the work through a coating or impregnation tank, air that will not readily become displaced from the work surface, and the lack of pressurizing means to force the fluid against the solid surface.
The patent literature and commercial applications recognize the problem and address it in various ways. Ultrasonic means for tinning wire in U.S. Pat. No. 3,685,487 employs magnetostrictive transducers to agitate a bath of tin alloy by vibration of the bottom of the tank. The fluid is put into motion which improves contact with the moving wire. U.S. Pat. No. 3,330,680 to Knapp also involves the coating of wire in a tank agitated by ultrasound.
U.S. Pat. Nos. 3,393,661; 3,401,542; 3,422,796; 3,511,730; 3,827,397; 4,020,196; 4,025,671; 4,059,068; all teach improved impregnation means by the use of grooves of different shapes in surfaces over which the impregnant passes. Impregnation fluid is forced through the grooves to contact the underside of the passing material.
Certain restrictions exist in such application methods, due to limited practical flow pressure through orifices. In the case of small diameter fibers such as 0.0005" graphite or glass fiber, the danger of breaking strands, and causing expansion of the bundle of fibers due to pressure, is ever present and highly undesirable, particularly in filament winding applications. Resinous materials used for reinforcing composite fibers are also high in viscosity and do not pass through a fiber bundle readily, as is necessary to displace air entrapped in the center. To increase the penetration difficulty, most fibers used for weaving and filament winding are twisted and sized to prevent fuzzing during the mechanical manipulations.
Existing process means for resinous impregnation of fibrous materials typically require a series of freely rotating rollers immersed in a tank of impregnating resin, over which the fiber passes, so that bending and squeezing forces tend to void the entrapped air, and to develop contact of the resin with the fibre. Speed through the bath is an economic consideration but is limited by the resin viscosity and drag on the fiber due to adhesion of the resin on the rollers.